24 



PROVISIONAL METHODS FOB ANALYSIS OF FOODS. 



melt the sample of fat and pour the clear fat through a filter, allowing the first two or 

 three drops to fall on the surface of the prism contained in casing B (oils must be 

 filtered if turbid) . For this purpose the apparatus should be. raised with the left 

 hand, so as to place the prism surface in a horizontal position. Then press B against 

 A and bring F back into its original position by turning it in the opposite direction. 

 Adjust the mirror until it gives the sharpest reading. If the reading be not distinct 

 after running water of a constant temperature through the instrument for some time, 

 the fat is not evenly distributed on the surfaces of the prism and the process must 

 be repeated. The instrument should be carefully adjusted by means of the standard 

 fluid which is supplied. As the index of refraction is greatly affected by tempera- 

 ture, care must be used to keep it constant. 



The following table can be used to convert the degrees of the instrument into 

 refractive indices: 



Butyro-r efractometer readings and indices of refraction. a 



b Winton, Conn. Expt. Sta., Rept., 1900, pt. 2, p. 143. 



4. DETERMINATION OF IODIN ABSORPTION, HUBL'S METHOD.* 



(a) PREPARATION OF REAGENTS. 



lodin solution. Dissolve 25 grams of pure iodin in 500 cc of 95 per cent alcohol. 

 Dissolve 30 grams of mercuric chlorid in 500 cc of 95 per cent alcohol. The latter 

 solution, if necessary, is filtered, and/then the two solutions are mixed. The mixed 

 solution should be allowed to stand twelve hours before using. 



Decinormal sodium thiosulfate solution. Dissolve 24.8 grams of chemically pure 

 sodium thiosulfate freshly pulverized as finely as possible and dried between filter or 

 blotting paper, and dilute with water to 1 liter at the temperature at which the 

 titrations are to be made. 



Starch paste. One gram of starch is boiled in 200 cc of distilled water for ten min- 

 utes and cooled to room temperature. 



Solution of potassium iodid. One hundred and fifty grams of potassium iodid are 

 dissolved in water and made up to 1 liter. 



Solution of potassium bichromate. Dissolve 3.874 grams of chemically pure potas- 

 sium bichromate in distilled water and make the volume up to 1 liter at the tempera- 

 ture at which the titrations are to be made. The bichromate solution should be 

 checked against pure iron. 



U. S. Dept. of Agr., Div. of Chem., Bui. 4(1 revised, p. 



